1. Field of the Invention
Embodiments described herein relate to detecting and classifying particles in a liquid using multi-angle-light-scattering (MALS), and in particular to monitoring of water for normal concentration levels of bacteria, including Heterotrophic Plate Count (HPC) bacteria, that are found in water for human consumption.
2. Background of the Invention
A major concern for municipal and commercial water treatment facilities is the detection and control of pathogenic microorganisms, both known and emerging, in potable water treatment and distribution. In addition, there may exist levels of Heterotrophic Plate Count (HPC) bacteria that must not be allowed to exceed standards. In addition, there are not only a number of chlorine resistant pathogens such as Cryptosporidium that can contaminate drinking water systems, but also potentially harmful microorganisms that can be introduced, either accidentally or intentionally, and propagate under suitable environmental conditions. Due to the length of time for standard laboratory methods to yield results, typically 24-72 hours, there has not been a reliable system to detect microbial levels in real-time and on-line to provide the water system operator with timely information on bacterial levels present in the water. Because of these expanding challenges, there has been an accelerated development of rapid tests and real-time methods to address the pressing needs of the water treatment community.
Conventional microbiological methods can be used to detect some of the microorganisms; however, such methods provide limited results. Analytical methods in microbiology were developed over 120 years ago and are very similar today. These methods incorporate the following steps: sampling, culturing and isolating the microbes in a suitable growth media by incubation, identifying the organisms through microscopic examination or stains, and quantifying the organisms. Cryptosporidium and Giardia form oocysts or cysts and cannot easily be cultured in conventional ways. To detect these protozoan pathogens, an amount of water containing suspected pathogens, typically 10 liters, is sent through a special filter to collect and concentrate the organisms. Then the filter is eluted and the organisms further processed by staining the organisms and sending the concentrated solution through flow cytometry for example. These procedures, which can be found in Standard Methods or ASME, require ascetic technique in sampling and handling, skilled technicians to perform the analysis, and a number of reagents, materials, and instruments to obtain results. Practically, such methods have proved to be time consuming, costly, and of little effectiveness for many current environmental field applications.
In order to reduce the amount of time to access microbiological results, a number of methods have been developed, mostly in the field of medicine. These faster tests have been improved and adapted to the environmental field and are generally categorized as 1) accelerated and automated tests 2) rapid tests and 3) contamination warning systems (CWS).
Accelerated tests are by grab sample and results can be obtained in 4 hours to 18 hours. Accelerated tests include immunoassays, ATP luminescence, and fluorescent antibody fixation. Rapid tests are also by grab sample and require manipulation of the sample to ‘tag’ the microbes with an identifiable marker or concentrate the microbe's genetic material (DNA) for subsequent identification. Results are normally available in 1-3 hours. These types of tests include Polymerase Chain Reaction (PCR) and Flow Cytometry.
Real time bacterial monitoring systems are continuous devices that detect levels of bacteria within a few minutes and may include laser based multi-angle light scattering (MALS) or multi-parameter chemical & particle instruments that detect water quality changes inferring potential biological changes. Continuous, real time detection of pathogens in water surveillance was first tried in the late 1960's and has progressed through a series of development steps until the first public field demonstration in 2002.
When light strikes a particle a characteristic scattering pattern is emitted. The scattering pattern encompasses many features of the particle including the size, shape, internal structures (morphology), particle surface, and material composition. Each type of microorganism will scatter light giving off a unique pattern herein called a Bio-Optical Signature. In traditional MALS, photo-detectors collect the scattered light and capture the patterns, which are then sent to a computer for analysis.
In addition to detecting both pathogenic and HPC bacteria in the water that occur naturally or are introduced intentionally, it is desirable to also monitor for the presence of blooms of bacteria, or biofilm slough-off, which are short-term events that normally are not caught by the grab-sample methods.
Presently, a detection system capable of meeting all of the ‘ideal detection system’ parameters, e.g., as cited by the American Water Works Association does not exist. Conventional devices and methods often differ in the amount of time to obtain results, degree of specificity, sampling frequency, concentration sensitivity, operating complexity, and cost of ownership.